JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS, VOL. 118, 2030–2044, doi:10.1002/jgre.20157, 2013 Insights into the subsurface structure of the Caloris basin, Mercury, from assessments of mechanical layering and changes in long-wavelength topography Christian Klimczak,1 Carolyn M. Ernst,2 Paul K. Byrne,1 Sean C. Solomon,1,3 Thomas R. Watters,4 Scott L. Murchie,2 Frank Preusker,5 and Jeffrey A. Balcerski 6 Received 15 June 2013; revised 7 September 2013; accepted 11 September 2013; published 3 October 2013. [1] The volcanic plains that fill the Caloris basin, the largest recognized impact basin on Mercury, are deformed by many graben and wrinkle ridges, among which the multitude of radial graben of Pantheon Fossae allow us to resolve variations in the depth extent of associated faulting. Displacement profiles and displacement-to-length scaling both indicate that faults near the basin center are confined to a ~ 4-km-thick mechanical layer, whereas faults far from the center penetrate more deeply. The fault scaling also indicates that the graben formed in mechanically strong material, which we identify with dry basalt-like plains. These plains were also affected by changes in long-wavelength topography, including undulations with wavelengths of up to 1300 km and amplitudes of 2.5 to 3 km. Geographic correlation of the depth extent of faulting with topographic variations allows a first-order interpretation of the subsurface structure and mechanical stratigraphy in the basin. Further, crosscutting and superposition relationships among plains, faults, craters, and topography indicate that development of long-wavelength topographic variations followed plains emplacement, faulting, and much of the cratering within the Caloris basin. As several examples of these topographic undulations are also found outside the basin, our results on the scale, structural style, and relative timing of the topographic changes have regional applicability and may be the surface expression of global-scale interior processes on Mercury. Citation: Klimczak, C., C. M. Ernst, P. K. Byrne, S. C. Solomon, T. R. Watters, S. L. Murchie, F. Preusker, and J. A. Balcerski (2013), Insights into the subsurface structure of the Caloris basin, Mercury, from assessments of mechanical layering and changes in long-wavelength topography, J. Geophys. Res. Planets, 118, 2030–2044, doi:10.1002/jgre.20157. 1. Introduction Guest,1975;Murchie et al., 2008; Watters et al., 2009a], which, after their emplacement, were subjected to extensional [2] The Caloris basin, with a diameter of ~1550 km the and contractional brittle deformation as well as changes in largest well-preserved impact basin on Mercury (Figure 1a) long-wavelength topography [Zuber et al., 2012]. The brittle [Murchie et al., 2008], preserves a complex tectonic history. fi deformation is evident by the juxtaposition of normal- and The interior of the basin is lled with smooth volcanic plains thrust-fault-related landforms [Strom et al., 1975; Melosh units [Murray et al., 1975; Strom et al., 1975; Trask and and McKinnon,1988;Watters et al., 2005, 2009a; Murchie et al., 2008]. Additional supporting information may be found in the online version of [3] The Mercury Dual Imaging System (MDIS) [Hawkins this article. et al., 2007] on the MErcury Surface, Space ENvironment, 1 Department of Terrestrial Magnetism, Carnegie Institution of Washington, GEochemistry, and Ranging (MESSENGER) spacecraft ac- Washington, D.C., USA. fi 2The Johns Hopkins University Applied Physics Laboratory, Laurel, quired the rst full images of the Caloris basin during its initial Maryland, USA. Mercury flyby in 2008. MDIS data revealed that the basin’s 3Lamont-Doherty Earth Observatory, Columbia University, Palisades, interior smooth volcanic plains units have spectral properties New York, USA. 4 distinct from those of plains units exterior to the basin Center for Earth and Planetary Studies, National Air and Space [Murchie et al., 2008; Robinson et al., 2008]. Further, spectral Museum, Smithsonian Institution, Washington, D.C., USA. 5German Aerospace Center, Institute of Planetary Research, Berlin, differences within the Caloris smooth plains were found to be Germany. present in ejecta blankets, floors, and central peaks of several 6Department of Earth, Environmental, and Planetary Sciences, Case impact craters, suggesting that those impacts had excavated Western Reserve University, Cleveland, Ohio, USA. distinct material from depth [Murchie et al., 2008; Ernst Corresponding author: C. Klimczak, Department of Terrestrial Magnetism, et al., 2010]. From estimates of the excavation depths of spec- Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., trally distinct materials exposed in crater deposits, Ernst et al. Washington, DC 20015-1305, USA. ([email protected]) [2010] derived a stratigraphy for several plains-covered areas ©2013. American Geophysical Union. All Rights Reserved. on Mercury and concluded that the interior plains in the central 2169-9097/13/10.1002/jgre.20157 part of the Caloris basin are between 2.5 and 4 km thick. 2030 KLIMCZAK ET AL.: TECTONICS OF THE CALORIS BASIN Figure 1. Overview of the Caloris basin area. (a) Location of the Caloris basin (black outline) relative to major expanses of smooth plains on Mercury (shown in blue, modified from Denevi et al. [2013]). (b) Digital terrain model (DTM) of the topography [Preusker et al., 2011] of the Caloris basin and the region to its west overlaid on a Mercury Dual Imaging System (MDIS) monochrome base map, in north polar stereographic projection centered at 140°E. Indicated in white are craters that show a notable tilt of their floors; arrows indicate the direction of tilt, but arrow length does not scale to tilt amount. The systematic tilts of crater floors away from topographic highs constrain the timing of long-wavelength topographic undulations and assist in the mapping of crests and troughs. M = Mozart basin; R = Raditladi basin. The location of the profile in Figure 1c is shown as a black, partially dashed line. (c) Profile A–A′ shows that topographic undulations can be approximately represented by a sinusoidal curve with a wavelength of 1300 km and an amplitude of 3 km. [4] Stereophotogrammetric analysis of MESSENGER flyby The first MESSENGER flyby revealed graben of basin-radial images yielded the first topographic information for the Caloris and -concentric orientations [Murchie et al., 2008], including basin [Oberst et al., 2010] and revealed long-wavelength the prominent complex of radial graben in the center of the basin, undulations of the basin floor (Figure 1b). Orbital observations now named Pantheon Fossae. There is no consensus yet for the with MESSENGER’s Mercury Laser Altimeter (MLA) instru- origin of Pantheon Fossae [Head et al., 2008, 2009; Freed et al., ment [Cavanaugh et al., 2007] confirmed these long-wavelength 2009; Watters et al., 2009a; Klimczak et al., 2010], but the fault topographic undulations [Zuber et al., 2012] and documented geometric analysis of Klimczak et al. [2010] suggested that the that the floors of several young, flat-floored impact craters graben-bounding faults are vertically confinedtostratigraphic within and around the basin are not horizontal but instead or mechanical units in the central Caloris basin. tilt in the same direction as the regional trend of the long- [6] Orbital images with substantially improved lighting wavelength topography (Figure 1). These observations sug- geometry and resolution (Figures 2 and 3) now permit an in- gest that topographic changes affected the area in and outside depth mechanical analysis of the faults of the Pantheon the basin after the emplacement of the plains, with parts of the Fossae graben. Such fault analysis can improve the character- basin interior units displaced to elevations greater than those ization of the depth extent of faulting throughout the Caloris of the basin rim. basin and can test whether there is a vertical confinement of [5] After their emplacement, the Caloris interior smooth the graben in the basin center. Information on the depth extent plains were affected by extensional and compressional tectonic of faulting and the confinement of the faults to mechanical or stresses, manifest by the many normal- and thrust-fault-related stratigraphic layering provides insight into the emplacement of landforms throughout the basin (i.e., graben and wrinkle the interior volcanic plains as well as the timing and processes ridges). Images returned by the Mariner 10 spacecraft, which involved in the changes in long-wavelength topography across viewed less than half of the basin, showed wrinkle ridges with the region. In this paper we characterize the shapes of the orientations both radial and concentric to the basin center, as graben-bounding faults within the Caloris basin in terms of well as graben of no preferred orientation that outline polygo- their lengths and displacements, infer the depth extent of nal blocks in the plains [Strom et al., 1975; Melosh and faulting, and compare these results to the topography within McKinnon,1988;Thomas et al., 1988; Watters et al., 2005]. and surrounding the Caloris basin. 2031 KLIMCZAK ET AL.: TECTONICS OF THE CALORIS BASIN Figure 2. MDIS monochrome base map of the Caloris basin interior (250 m/pixel). Note the prominent basin-radial graben (Pantheon Fossae) in the center of the basin, and the basin-circumferential graben far- ther from the center. Also, note the many thrust-fault-related landforms throughout Caloris. The graben used for the displacement analysis